BEIRUT — Syrian President Bashar Assad made his first appearance on state TV in nearly three weeks Tuesday in a show of solidarity with a senior Iranian envoy, even as the U.S. urged stepped up international planning for the regime’s collapse.

The visit to Damascus by the highest-ranking Iranian official since the uprising began coincided with a warning by an increasingly agitated Tehran that it holds the U.S. responsible for the fate of 48 Iranians seized by Syrian rebels.

Appearing together on state TV, Assad and Iran’s Saeed Jalili vowed to defeat the rebels and their backers, while U.S. Secretary of State Hillary Rodham Clinton predicted Assad’s regime was quickly unraveling, with high-level defections such as his prime minister’s switch to the rebel side.

Jalili’s visit highlighted Assad’s deepening reliance on a shrinking list of allies, led by Tehran.

On a visit to South Africa, Clinton urged international leaders to begin work on a “good transition plan” to try to keep Syria from collapsing into more chaos after Assad.

The rebels seized the 48 Iranians in a bold daylight attack near Damascus on Saturday, claiming they were members of Iran’s powerful Revolutionary Guards sent on a “reconnaissance mission” to assist in Assad’s crackdown. Iran says they were pilgrims .

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